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Hi I know there has been alot of discussion on over heating crf250r's, but I have a bit of a stumper.  My 250r started spewing radiator fluid out the overflow and was getting really hot, riding about 25 mph on open trails after about 5 min.  After reading many posts on this site and others we figured a blown head gasket or warped head so we disassembled it, had the cylinder and head checked for warp at a machine shop, there was none, replaced both gaskets, and re-assembled.  Bike starts up easy but gets too hot too fast.  Our comparison is our 2004 crf250r running at the same time for comparison.  I opened up the radiator cap and re-started, and noticed alot of air in the coolant in the 2006 that is overheating where there really isn't any in the 2004 for comparison.  So, I believe exhaust air is getting into the coolant and heating / pressurizing it, but the gaskets were just replaced (OEM).  I didn't notice any cracks in the head, and there is no coolant leaking from the radiator fins.  Also swapped out the radiator cap with the 2004, didn't help.

 

Any suggestions on what to check next?

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How's the water pump?  I'd check that and I'd change the coolant as well (which you need to drain to get to the water pump anyways). It could be shot and not circulating the coolant like it should, and coolant is just at a stand still inside the engine and radiators. 

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Thanks for the reply, I forgot to mention that we just rebuilt the water pump, and verified the coolant is circulating.  The machinist at the machine shop that checked the heads for warp suggested we change out the cylinder studs because they may not be torquing down correctly.  Rebuilt the water pump due to coolant coming out the water pump weep hole, replaced the oil and water seals, bearings, shaft and impeller.  Also checked for blockages in the radiators and cylinder flow ports for coolant and they are all clear.  Everything torqued to specs in re-assembly.

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SOANS, I didn't try a new rad cap but tried the cap from our 2004 crf250r that has no issues.  Just can't figure out how air is getting into the coolant.

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Dude, sounds like you've got air in the coolant. What I usually do is fill the radiator with coolant, then squeeze all the hoses multiple times, this helps push bubbles out. Think about it, if your coolant has air in it, then your pump may be taking up air and not pushing coolant, not to mention the air is taking up space where coolant should be. If your machinist said the head checked out, and you put a new head gasket, you should not have any exhaust getting in the coolant system. Does your bike have an overflow tank?

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Asp1227, makes sense, I'll try to see if I can get any additional air bubbles out.  I did tip it from side to side after filling the radiators to try and get air bubbles out, but didn't really work the hoses like you suggest, thanks for the suggestion

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ASP1227, looks like you were right.  I squeezed the air out of the hoses, topped off the coolant, started it up and no more bubbles in the radiator.  The engine ran significantly cooler, maxed out about 200 F after riding for about 30 min.  where it had hit over 200 F in less than 5 min when it was pushing air bubbles in the coolant.  The water pump impeller must have grabbed an air bubble and foamed the coolant which allowed the motor to overheat.

Thanks again for the help, this is a great forum !

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